Ryan McKiernan*, Lydia Strinati, Vanessa Ansah-Pewudie, Soha Jamshed Iqbal and Maryam Rida Asad
Cepaea nemoralis, commonly known as the brown-lipped snail, is native to Western Europe. It is a model organism when undertaking evolutionary biology studies as it displays a high frequency of polymorphism. By examining the various shell patterns and colours it is possible to determine if this polymorphism is due to selective pressures, genetic drift or human sampling error.
Snails are an ideal organism to study polymorphism through as they have a short life-cycle, rarely move further than 20m from their native habitat preventing a large amount of gene flow between separate populations and are easily accessible to researchers.
In this study, we will take a total of six samples across a horizontal transect all at the same altitude. Three of the samples will come from areas of hedges and the other three will come from areas of scrub, with each sample site at least 20m apart. By taking a large amount of samples, the likelihood of the results showing human sampling error is decreased and because these samples are taken from the same altitude, this eliminates that particular variable from consideration. At the end of the study, at least 120 snails should have been examined and their life stage, shell colour and number of bands noted. Analysing these results will allow you to determine the cause behind the polymorphism displayed.
If a significant difference is found between your results then it can be said that either selection or genetic drift is acting on the snails. If the snails are under selective pressures, their shell colour should be similar to their environment so that snails found in the hedges will have darker, unbanded shells while snails in the grassland/scrub will have lighter, banded shells allowing shadows to be cast on them (Minkoff, 1983). Evidence like this would support that the selective pressure of predation is acting on the snails. However, it is also possible to say that genetic drift is acting on the snails and that this polymorphism simply exists due to evolutionary chance. If you find a gradual change in shell colour and banding across the horizontal transect, paying particular attention to the boundaries of your sample sites, you can conclude that genetic drift could be occurring. Jones et al (1977), reminds us that when a population decreases rapidly, a bottleneck effect can occur, allowing genetic drift to act quickly on the small population. A bottleneck effect would also potentially give rise to pseudo-replication, a statistical error generated by considering interdependent snail populations as independent ones. These are all factors that must be taken into account during this study and when analyzing the results.
The null hypothesis suggests that there will be no change in phenotypes across our sites and this would be true if all the samples collected were identical. The alternative hypothesis suggests that there will be a change in snail phenotypes as polymorphism exists and therefore selection, genetic drift or human sampling error has given rise to these results.
Minkoff, E. C., 1983. Evolutionary Biology. 1st ed. Reading: Addison-Wesley.
Jones, J.S., Leith, B.S., Rawlings, P. 1977. Polymorphism in Cepaea: A Problem with Too Many Solutions? Annual Review of Ecology and Systematics, Vol. 8, 109-43.
Cepaea nemoralis, commonly known as the brown-lipped snail, is native to Western Europe. It is a model organism when undertaking evolutionary biology studies as it displays a high frequency of polymorphism. By examining the various shell patterns and colours it is possible to determine if this polymorphism is due to selective pressures, genetic drift or human sampling error.
Snails are an ideal organism to study polymorphism through as they have a short life-cycle, rarely move further than 20m from their native habitat preventing a large amount of gene flow between separate populations and are easily accessible to researchers.
In this study, we will take a total of six samples across a horizontal transect all at the same altitude. Three of the samples will come from areas of hedges and the other three will come from areas of scrub, with each sample site at least 20m apart. By taking a large amount of samples, the likelihood of the results showing human sampling error is decreased and because these samples are taken from the same altitude, this eliminates that particular variable from consideration. At the end of the study, at least 120 snails should have been examined and their life stage, shell colour and number of bands noted. Analysing these results will allow you to determine the cause behind the polymorphism displayed.
If a significant difference is found between your results then it can be said that either selection or genetic drift is acting on the snails. If the snails are under selective pressures, their shell colour should be similar to their environment so that snails found in the hedges will have darker, unbanded shells while snails in the grassland/scrub will have lighter, banded shells allowing shadows to be cast on them (Minkoff, 1983). Evidence like this would support that the selective pressure of predation is acting on the snails. However, it is also possible to say that genetic drift is acting on the snails and that this polymorphism simply exists due to evolutionary chance. If you find a gradual change in shell colour and banding across the horizontal transect, paying particular attention to the boundaries of your sample sites, you can conclude that genetic drift could be occurring. Jones et al (1977), reminds us that when a population decreases rapidly, a bottleneck effect can occur, allowing genetic drift to act quickly on the small population. A bottleneck effect would also potentially give rise to pseudo-replication, a statistical error generated by considering interdependent snail populations as independent ones. These are all factors that must be taken into account during this study and when analyzing the results.
The null hypothesis suggests that there will be no change in phenotypes across our sites and this would be true if all the samples collected were identical. The alternative hypothesis suggests that there will be a change in snail phenotypes as polymorphism exists and therefore selection, genetic drift or human sampling error has given rise to these results.
Minkoff, E. C., 1983. Evolutionary Biology. 1st ed. Reading: Addison-Wesley.
Jones, J.S., Leith, B.S., Rawlings, P. 1977. Polymorphism in Cepaea: A Problem with Too Many Solutions? Annual Review of Ecology and Systematics, Vol. 8, 109-43.